f2fs: introduce large directory support

This patch introduces an i_dir_level field to support large directory.

Previously, f2fs maintains multi-level hash tables to find a dentry quickly
from a bunch of chiild dentries in a directory, and the hash tables consist of
the following tree structure as below.

In Documentation/filesystems/f2fs.txt,

----------------------
A : bucket
B : block
N : MAX_DIR_HASH_DEPTH
----------------------

level #0   | A(2B)
           |
level #1   | A(2B) - A(2B)
           |
level #2   | A(2B) - A(2B) - A(2B) - A(2B)
     .     |   .       .       .       .
level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
     .     |   .       .       .       .
level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)

But, if we can guess that a directory will handle a number of child files,
we don't need to traverse the tree from level #0 to #N all the time.
Since the lower level tables contain relatively small number of dentries,
the miss ratio of the target dentry is likely to be high.

In order to avoid that, we can configure the hash tables sparsely from level #0
like this.

level #0   | A(2B) - A(2B) - A(2B) - A(2B)

level #1   | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
     .     |   .       .       .       .
level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
     .     |   .       .       .       .
level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)

With this structure, we can skip the ineffective tree searches in lower level
hash tables.

This patch adds just a facility for this by introducing i_dir_level in
f2fs_inode.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

1 files changed, 12 insertions, 9 deletions

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index c3ea8f8cc80a..582fa00f3597 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -21,12 +21,12 @@ static unsigned long dir_blocks(struct inode *inode)
                                                         >> PAGE_CACHE_SHIFT;
 }
 
-static unsigned int dir_buckets(unsigned int level)
+static unsigned int dir_buckets(unsigned int level, int dir_level)
 {
         if (level < MAX_DIR_HASH_DEPTH / 2)
-                return 1 << level;
+                return 1 << (level + dir_level);
         else
-                return 1 << ((MAX_DIR_HASH_DEPTH / 2) - 1);
+                return 1 << ((MAX_DIR_HASH_DEPTH / 2 + dir_level) - 1);
 }
 
 static unsigned int bucket_blocks(unsigned int level)
@@ -65,13 +65,14 @@ static void set_de_type(struct f2fs_dir_entry *de, struct inode *inode)
         de->file_type = f2fs_type_by_mode[(mode & S_IFMT) >> S_SHIFT];
 }
 
-static unsigned long dir_block_index(unsigned int level, unsigned int idx)
+static unsigned long dir_block_index(unsigned int level,
+                                int dir_level, unsigned int idx)
 {
         unsigned long i;
         unsigned long bidx = 0;
 
         for (i = 0; i < level; i++)
-                bidx += dir_buckets(i) * bucket_blocks(i);
+                bidx += dir_buckets(i, dir_level) * bucket_blocks(i);
         bidx += idx * bucket_blocks(level);
         return bidx;
 }
@@ -143,10 +144,11 @@ static struct f2fs_dir_entry *find_in_level(struct inode *dir,
 
         f2fs_bug_on(level > MAX_DIR_HASH_DEPTH);
 
-        nbucket = dir_buckets(level);
+        nbucket = dir_buckets(level, F2FS_I(dir)->i_dir_level);
         nblock = bucket_blocks(level);
 
-        bidx = dir_block_index(level, le32_to_cpu(namehash) % nbucket);
+        bidx = dir_block_index(level, F2FS_I(dir)->i_dir_level,
+                                        le32_to_cpu(namehash) % nbucket);
         end_block = bidx + nblock;
 
         for (; bidx < end_block; bidx++) {
@@ -467,10 +469,11 @@ start:
         if (level == current_depth)
                 ++current_depth;
 
-        nbucket = dir_buckets(level);
+        nbucket = dir_buckets(level, F2FS_I(dir)->i_dir_level);
         nblock = bucket_blocks(level);
 
-        bidx = dir_block_index(level, (le32_to_cpu(dentry_hash) % nbucket));
+        bidx = dir_block_index(level, F2FS_I(dir)->i_dir_level,
+                                (le32_to_cpu(dentry_hash) % nbucket));
 
         for (block = bidx; block <= (bidx + nblock - 1); block++) {
                 dentry_page = get_new_data_page(dir, NULL, block, true);